Complement component 3, often simply called C3, is a protein of the immune system. It plays a central role in the complement system and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.^[1][2]

Function

C3 plays a central role in the activation of complement system.^[3] Its activation is required for both classical and alternative complement activation pathways. People with C3 deficiency are susceptable to bacteria infection.^[4][5]

Soluble C3-convertase, also known as C4b2a, catalyzes the proteolytic cleavage of C3 into C3a and C3b as part of the classical complement system as well as the mannan-binding lectin pathway. C3a is an anaphylotoxin, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by iC3Bb, another form of C3-convertase.

Structure

Several crystallographic structures of C3 have been determined and reveal that this protein contains 13 domains.^[6][7][8][9]

Clinical use

Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with some types of kidney disease such as post-infectious glomerulonephritis and shunt nephritis.

References

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